X-ray collimator grids assist one to obtain a clear image of a distant X-ray source. The x-rays travel toward the X-ray detector in a straight line. X-rays propagating from the source pass through the collimator tube and grids while X-rays arriving from other directions which would degrade the desired image are blocked. The collimator grid allows parallel X-rays traveling in parallel to the collimator to pass through to an X-ray target upon which the X-ray image is formed. The arrangement is somewhat analogous to the light sorting accomplished by ordinary "Venetian" blinds, such as found in one's household windows. Formed of spaced, flat, parallel light impervious slats, the Venetian blinds sorts light rays. When the slats in the blinds are oriented perpendicular to the glass window, the image enters from the front and passes through. However, stray light arriving from directions higher up or lower down is blocked by the slats.
One prior collimator arrangement for sorting X-rays is disclosed in a prior U.S. patent to Delhumeau, U.S. Pat. No. 2,605,427 granted Jul. 29, 1952, presenting a grid device for preventing diffusion of X-rays from a nearby X-ray source. Delhumeau mounts slats of heavy metal, such as Lead, a material that is impermeable to X-radiation, within slots or grooves, however termed, formed in a resin support structure, a material permeable to X-radiation, spacing the slats about 0.4 mm apart. The grooves are about 1.5 mm deep and about 0.1 mm wide.
Because of the close proximity of the X-ray source in Delhumeau's system, the rays from the source travel in a path defining a right circular cone toward Delhumeau's focusing device and, accordingly, the metal slats are oriented, not in parallel, but at progressively smaller angles relative to the face of his device in dependence upon the distance of the slat from the axis of the X-ray source. Delhumeau's grid thus "focalizes" the oncoming X-radiation, unlike the present invention, which collimates the X-radiation.
The Delhumeau patent also hypothesizes alternative forms for the heavy metal, suggesting disposition of a metal powder in the grooves, or, with modification of the frame, an absorbent liquid, such as Mercury, but offers few details for implementation. For a Silver amalgam, Delhumeau notes that the amalgam hardens over time. Notwithstanding those hypotheticals, one recognizes that the anti-diffusion grid structure of Delhumeau is perhaps intended for medical or industrial application having close by X-ray sources and not for unattended use in exploration in outer space.
In a prior patent to Frazier et al, U.S. Pat. No. 5,416,821, granted May 16, 1995 and assigned to TRW Inc., the assignee of the present invention, a novel X-ray collimator grid is described that is useful even in unattended space exploration. Frazier found that a monocrystalline silicon wafer affords a robust and effective collimator support structure that withstands the rigors of the low temperature vacuum regions of outer space as well as the transition from earth atmosphere to that environment and back. Frazier's grid contains an array of spaced parallel heavy metal (high "Z") slats, impervious to X-radiation, that are mounted upon a silicon crystal substrate or, as variously termed, wafer, an X-ray permeable material. Suitable heavy metals are those having an atomic weight equal to or greater than the atomic weight of Hafnium, where Z equals 72, such as Tungsten of a Z equal to 74. The slats in that structure are oriented in parallel with the &lt;111&gt; crystal plane.
To fabricate Frazier's collimator structure, the silicon crystal slab is etched to create a number of voids or apertures within a central region of the wafer that extend through the crystal wafer. Those apertures are separated by retained portions of the silicon crystal and define ribs or straight frame sections that extend across that central region. Grooves, trenches or slots, as variously termed, of microscopic sized widths, typically in the range of fifteen microns through one-hundred microns in width, are etched into the silicon crystal wafer oriented in parallel with the &lt;111&gt; crystal plane and seat the heavy metal slats.
The foregoing construction produced a majority of linear slots that are discontinuous due to the intervening apertures, extending in a linear path across the remaining portions of the silicon slab, including the laterally extending silicon ribs bounding the apertures. Straight flat slats formed of a heavy metal, Tungsten, as example, were then picked up and manually inserted within the respective trenches or slots as could be accomplished with vacuum tweezers. The apertures through the silicon wafer provided clearance space for handling and inserting the metal slats, although being located in the path of the slots created the physical discontinuity or gaps in the slot's linear extent. The monocrystalline silicon wafer was oriented so that the face of the crystal was in the &lt;1,1,0&gt; crystallographic plane to permit proper etching of deep narrow slots. For additional details of fabrication and application of that collimator grid and as additional background, the reader is invited to refer to and review the Frazier et al patent.
In practice, the Frazier et al structure proved difficult to manufacture. It was found that Tungsten, though strong and stiff, was difficult to form into the microscopically thin strips or foils having the requisite flatness, and the foils surface was uneven. Because of the Tungsten slat's essentially rippled surface, the slats would not easily fit into the slots, making assembly difficult. When forced into the slot, a slat often would damage the side walls of the slot and the slot thereafter could no longer reliably support the respective slat. As a consequence, the yield of collimators was prospectively low, and the manufacturing expense anticipated was higher than desired.
Lead, which oxidizes, had other difficulties that were thought to make that material undesirable for slats in the X-ray collimator application. Because of the difficulties with the foregoing metals, resort was made to another heavy metal for the slats, Gold. The gold does not corrode and is much softer than Tungsten or Lead, which makes it desirable, but, it is more expensive than the latter materials.
Although Gold could be produced in straight flat strips at the fifteen micron thickness level, the strips did not have sufficient rigidity. In effect, in the elongated form of a microscopically thin slat, the Gold was found too soft and limp. Thus Gold slats also proved difficult to mechanically insert in a straight line in the discontinuous sections of a microscopic slot or trench formed in the silicon wafer structure described in the Frazier et al patent. It is apparent, thus, that the process described by Frazier et al produced collimator grids that would have a higher than desired production cost. As an advantage, the present method invention does not require the laborious mechanical insertion of straight slats into microscopic slots.
Accordingly, an object of the present invention is to provide a new and more easily accomplished method of fabricating high Z metal slats within a silicon substrate X-ray collimator grid that avoids the requirement for pre-forming straight slats of heavy metals and avoids the step of mechanically inserting slats of heavy metal within slots formed in the surface of the silicon crystal substrate.
And an ancillary object of the invention is to provide a new rugged X-ray collimator grid structure, fabricated by the new method.